1. Field of the Invention
This invention has relation to cranes designed to pick up and deliver loads to and from a first platform such as an offshore oil rig, and a second platform such as a lighter or supply boat, during a run of heavy seas, for example. The cyclical vertical motion of the second platform with respect to the first, plotted against time closely approximates a sine wave.
2. Description of the Prior Art
In comparatively recent times, such thought has been given to the problem of using a crane mounted to a relatively stable platform for lifting a load from the deck of a supply ship or the like in heavy weather at precisely the right time so as to try to avoid the danger of damage to the load or cargo and/or to the supply ship, to try to minimize the possibility of destructive overloads on the crane and its accutrements mounted on the first platform, and, at the same time, to minimize the necessary weight, strength and power of the crane and the power equipment associated with it and its winches.
It is well known to use a crane mounted on a first platform such as an offshore oil rig and having its boom extending outwardly from the platform to position its boom point in vertical clearing relation to a second platform such as a supply vessel or lighter on which a load to be transferred to the first platform is located. Typically, such structure will have a main hoist drum on which is operably wound or mounted a main load line that extends up the boom and over an appropriate sheave at the boom point, the outer end of the load line carrying a main hook or other device for fastening to the load to be picked off of the second platform. In order to attempt to control the main line hook so that there is little or no relative movement with respect to the second platform, others have used a signal line or messenger line which extends from a signal line drum or winch on the first platform, up the boom, over the boom point, and down into a fastened or fixed relationship with respect to the second platform. Means have been provided for monitoring the difference in motion between the main hoist drum and the signal drum so that the main hoist drum motor can be operated so as to insure that the motion of the main hoist drum is in sympathy with the motion of the signal drum. See U.S. Pat. No. 4,132,387 to Somerville et al, granted in January of 1979, for example.
Other patents which disclose the use of some kind of a signal line held or secured with respect to the deck of a supply boat, and running back over a boom point of a crane boom situated on an offshore platform or the like are: